The present invention relates to the radio frequency (RF) signal handling arts. It finds particular application in conjunction with medical magnetic resonance imaging systems and will be described with particular reference thereto. It is to be appreciated, however, that the invention will also find application in conjunction with other types of magnetic resonance imaging systems, magnetic resonance spectroscopy systems, and the like, as well as in other arts with analogous radio frequency signal handling.
In magnetic resonance imaging, a strong uniform static magnetic field B0 is generated, often by a superconducting magnet. The static magnetic field B0 polarizes the nuclear magnetic spin system of an object to be imaged. A solenoid superconducting magnet generates the static magnetic field, B0, along its own longitudinal axis and the common longitudinal axis of the cylindrical bore of the vacuum vessel, commonly denoted as the z-axis. Alternately, the B0 field is generated in an open region between a pair of poles.
To generate a magnetic resonance signal, the polarized spin system is first excited by applying a magnetic resonance excitation signal or radio frequency magnetic field B1, perpendicular to the z-axis. This RF field B1 is typically produced by an RF coil located inside the bore of a bore-type magnet or adjacent the pole of an open magnet and closely conforming thereto to maximize the space available to receive a patient. The RF magnetic field is turned on and off to create short RF pulses to excite and manipulate magnetization in the polarized object in the bore. More specifically, the RF excitation pulses tip the magnetization out of alignment with the z-axis and cause its macroscopic magnetic moment vector to precess around the z-axis. The precessing magnetic moment, in turn, generates a radio frequency magnetic resonance signal. Additional RF pulses are commonly applied to manipulate the resonance to form enhanced signal strength RF echoes which are received by the same RF coil or a local RF coil positioned near a region of interest.
In magnetic resonance imaging, it is advantageous for the RF coil to have high sensitivity, high RF power efficiency, and a high signal-to-noise ratio. Also, the B1 magnetic field which is generated should be uniform. The sensitivity of the RF coil is defined as the magnetic field B1 created by a unit current. The signal-to-noise ratio is proportional to the sensitivity and to the square root of the coil quality factor, Q.
To encode a sample spatially, magnetic field gradients are applied concurrent with and after the RF excitation. The gradient magnetic fields are typically applied in pulses to generate magnetic field gradients Gx, Gy and Gz linearly along the x, y, and z-directions, respectively, or along other selected coordinate systems. The gradient pulses typically are generated by gradient magnetic field coils which are also located inside the bore of a bore-type magnet or adjacent the poles of an open-type magnet. Commonly, the gradient field coils are mounted in back of the RF coil in the bore or on the pole piece.
A sequence control circuit controls the gradient pulses and the transmitter to generate a plurality of imaging sequences. For the selected sequence, the receive coil receives one or a plurality of echoes in rapid succession following each RF excitation pulse. Gradient pulses are typically applied before and during reception of resonance echoes and between echoes in multi-echo sequences. An analog-to-digital converter converts each data line to a digital format. Ultimately, the radio frequency signals are demodulated by a receiver located remote from the magnetic field and reconstructed into an image representation by a reconstruction processor which applies a two-dimensional Fourier transform or other appropriate reconstruction algorithm. The image may represent a planar slice through the patient, an array of parallel planar slices, a three-dimensional volume, or the like.
Radio frequency coils are generally connected to the RF transmitter and/or the RF receiver of the magnetic resonance system using coaxial cable. Coaxial cable is designed to protect the system from picking up extraneous RF signals which are present in the environment. Coaxial cables feature a surrounding shield or ground conductor separated from a current carrying central conductor by a dielectric material. The surrounding ground conductor acts as a shield that minimizes the pick-up of foreign frequencies by the central conductor of the cable.
Although coaxial cable is used, there are still coupling problems at resonance frequencies, such as 63 MHz for hydrogen dipoles in a 1.5 T B0 field. Among other things, the shield conductor of the coaxial cable itself tends to carry foreign induced currents, such as from TV transmissions, stray harmonics from the gradient pulse oscillators and clocking circuits in nearby equipment, and the like. The induced current is often referred to as xe2x80x9cskin currentxe2x80x9d because it flows on the outside of the shield conductor. The stray RF current tends to flow out of the bore and into other circuits, such as the amplifiers, analog-to-digital converters, receivers, and reconstruction processor to contribute errors in the resultant image.
Balance/unbalance (xe2x80x9cBalunxe2x80x9d) circuitry is used as one means for reducing, or xe2x80x9ctrappingxe2x80x9d, the noise and/or stray RF currents generated due to induced currents in the coaxial cable. Baluns of the prior art consisted of an LC frequency filter for each cable located in a copper shielded box. The balun was tuned to the frequency of interest, such as by a tuning capacitor.
The baluns of the prior art were problematic for a number of reasons. First, the baluns were inserted in-line. Each coaxial cable was interconnected with a balun. As the number of RF channels increases, the number of coaxial connections becomes labor-intensive. Second, the baluns were space-consuming. Space limitations become more problematic as the number of channels increases and more baluns are necessary. In magnetic resonance scanners, there are severe space limitations. In bore type magnets, there is pressure to reduce the magnet diameter for lower cost competing with pressure to enlarge the patient receiving bore. Similarly, in openmagnets, there are competing pressures to move pole pieces closer and to enlarge the patient gap. This compresses the space available for RF coils, gradient coils, shims, baluns, and other associated structures. Third, the baluns were expensive due to the use of special non-magnetic tuning capacitors.
Compounding the aforementioned disadvantages, there is a consumer demand for magnetic resonance scanners with greater numbers of RF output channels, such as a channel for each quadrature mode, a channel for each individual coil or coil mode of an array, or the like. The multiple connection of parallel baluns reduces their effectiveness to block stray RF currents. Also, space consuming problems are magnified as the number of baluns increases. Finally, multiple baluns multiply the cost.
The present invention contemplates a new and improved design for trapping the noise and/or stray RF currents generated due to induced currents in the coaxial cable, which overcomes the above-referenced problems and others.
In accordance with one aspect of the present invention, a magnetic resonance apparatus is disclosed, having a main magnet which generates a main magnetic field through an examination region, at least one RF coil assembly positioned adjacent the examination region, a radio frequency transmitter which drives the RF coil assembly to excite magnetic resonance dipoles in a RF transmit field within the examination region, a multi-channel receiver connected by shielded transmission cables to at least one RF coil assembly which receives and demodulates the magnetic resonance signals, and further including a multi-channel RF cable trap inductively coupled to the shielded transmission cables to block stray induced RF currents and noise from entering the receiver.
In accordance with another aspect of the present invention, a multi-channel RF trap is disclosed for blocking stray signals on a plurality of shielded RF cables which each have a peripheral shield conductor and at least one inner conductor for carrying RF signals, the multi-channel RF trap comprising: a conductive layer inductively coupled to the shield conductors of the plurality of shielded RF cables; and a capacitive element connected with the conductive layer, the capacitive element being sized and the conductive layer being shaped such that the capacitive element and the conductive layer define an LC circuit with a resonance frequency at a frequency of RF signals carried on the inner conductor.
In accordance with another aspect of the present invention, a method of magnetic resonance is disclosed, comprising: exciting magnetic resonance in dipoles of a subject such that magnetic resonance signals are generated at least at a resonance frequency; picking up resonance signals and conveying the resonance signals on a center conductor of a cable, which center conductor is surrounded by a shield conductor; inductively attenuating signals on the shield conductor within a frequency spectrum that encompasses the resonance frequency; and demodulating the resonance signals from the center conductor.
In accordance with yet another aspect of the present invention, a method for trapping RF currents in a shielded cable carrying signals at a working RF frequency is disclosed, the method comprising inductively coupling a spatially distributed resonant electrical circuit in close proximity to the shielded cable, the resonant electrical circuit having a sufficiently high impedance at the working RF frequency to attenuate stray RF currents and noise in the cable.
One advantage of the present invention is that it minimizes the space required for the RF cable trap, especially in the case of a large number of cables.
Another advantage of the present invention is its multiple cable capacity.
Another advantage of the present invention resides in easier insertion and removal of cables. In-line cable connections are eliminated.
Yet another advantage of the present invention is that it is cost effective.
Still further advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description.